Many commercial, industrial and government facilities require a significant number of lighting fixtures for adequate illumination, and therefore use a significant amount of power to operate the fixtures. To reduce power consumed to light these facilities, a number of facilities use lighting control systems which control when the lighting fixtures are energized. For example, a step-dimming system, such as the two-level lighting control system disclosed in U.S. Pat. No. 5,216,333 to Nuckolls, can be used to switch facility fixtures between energy-saving low level or reduced-wattage operation and full level or normal-wattage operation in accordance with output signals from a motion sensor. Step-dimming systems can respond to other conditions besides occupancy level such as ambient light level, time, and manual switching. U.S. Pat. No. 4,713,598 to Smith discloses another device for controllably switching an AC line to energize a load. The device uses a passive infrared detector to sense motion.
Passive infrared wall switch sensors such as H-MOSS.RTM. motion switching system models 1500A and 750A, manufactured by Hubbell Incorporated of Bridgeport, Conn., can be used to automatically power up incandescent or fluorescent lighting fixtures upon detection of occupant motion, and to power down the fixtures after a predetermined period of time has elapsed following the last instance of detected motion. This period of time shall hereinafter be referred to as the time out (TO) period. These wall mounted switches provide adjustable TO periods, that is, the person installing the switch can specify the duration of the TO period by selecting and entering a numerical value corresponding to the desired duration of the TO period from a range of values. In addition, the 1500A model has a photocell device for controlling lighting fixtures in accordance with detected ambient light levels. In either model, the duration of the TO period remains fixed until it is manually adjusted by a person.
These lighting control devices are characterized by a number of drawbacks. For example, if the fixed TO period is too long, the wall switch does not realize maximum energy and cost savings. If the fixed TO period is too short, the wall switch powers down lighting fixtures while an occupant is still in the lighted area.